1. Field of the Invention
The present invention relates to a gas sensor composed of an oxygen ion conductive solid electrolyte, and specifically relates to a NOx sensor.
2. Description of the Background Art
Conventionally, various measuring devices have been used for determining a concentration of a desired gas component in a measurement gas. A known device of measuring a NOx concentration in a measurement gas such as a combustion gas, for example, is a NOx sensor formed of an oxygen ion conductive solid electrolyte, such as zirconia (ZrO2) (see Japanese Patent Application Laid-Open No. 8-271476, for example).
According to a NOx sensor disclosed in Japanese Patent Application Laid-Open No. 8-271476, O2 in a measurement gas introduced from outside is previously removed by pumping in the first internal space so as to make the measurement gas to be in a state of low oxygen partial pressure (a state in which oxygen partial pressure is lowered to the extent that the measurement of NOx is not affected by the presence of O2 in the measurement gas), and thereafter, the measurement gas is introduced into the second internal space. Then, NOx is reduced in the measuring electrode by applying a constant voltage between the measuring electrode containing such as Pt and Rh provided in the second internal space and the reference electrode provided in the air. The NOx concentration is to be detected on the basis of a value of a current flowing at that time between the measuring electrode and the reference electrode, the value being proportional to the NOx concentration.
In order to improve the accuracy of measurement in a sensor for measuring the NOx concentration in the aforementioned manner, O2 concentration in the second internal space is needed to be controlled accurately. To describe more in detail, when NOx does not exist in the measurement gas introduced into the second internal space, ideally, it is desirable that a current should not flow between the measuring electrode and the reference electrode. However, since O2 actually exists even in a small amount of oxygen partial pressure, a current (offset current, zero-point current) induced by decomposition of O2 flows when applying a voltage between the measuring electrode and the reference electrode. This offset current is to be superimposed on a current flowing when measuring the NOx concentration. Accordingly, the NOx concentration calculated in the aforementioned manner includes albeit only slightly an error attributed to this offset current. Therefore, the problem arises that the calculated NOx concentration is not necessarily accurate when the NOx concentration in the measurement gas is small.